The 555 timer IC:
 is an integrated circuit (chip) used in a variety of timer, delay, pulse generation, and oscillator applications. Derivatives provide two (556) or four (558) timing circuits in one package.^[2] The design was first marketed in 1972 by Signetics.^[3][4] Since then, numerous companies have made the original bipolar timers, as well as similar low-power CMOS timers. In 2017, it was said that over a billion 555 timers are produced annually by some estimates, and that the design was “probably the most popular integrated circuit ever made”.^[5]